This application claims the priority of British Patent Application No. 0007921.0 filed Mar. 31, 2000, the disclosure of which is hereby fully incorporated by reference herein.
The present invention relates to power factor correction, and more particularly to a power factor corrector for use with a power supply which provides a variable output voltage.
It is important to maximise the power factor of a circuit as any imbalance presented to the mains supply by industrial power users is metered by the electricity supply company.
The use of boost power factor correction circuits to address this problem is well known. However, it is not readily possible to reduce the output voltage of such circuits below that of the supply. Also, they can be unreliable as they require the use of electrolytic capacitors which tend to dry out and fail.
In order to minimize the harmonic distortion applied to the input supply, the corrector should aim to draw a current having a sine waveform synchronized with the supply, thereby presenting a balanced load. Furthermore, international regulations require that a power supply meets specific harmonic current limits.
A conventional approach to the problem of power factor correction is a two-stage process. One example of a two-stage process is shown in U.S. Pat. No. 5,003,453, to Tighe et al., in which each of the three phases is stepped up individually using a high power factor correction circuit controlled by feedback to generate a respective DC output voltage, and then each of these DC voltages is converted by a DC/DC converter back down to a desired level. The resultant three DC voltages are then combined in parallel.
Many conversion processes, including many two-stage conversion processes, have a control system which operates on the basis of the peak input voltage value, which is calculated from the rms voltage value. Clearly, this approach gives an incorrect result, however, because the peak measurement can only be obtained by using the result from the previous cycle of mains.
U.S. Pat. No. 5,731,969, to Small, shows an analogue system having three power factor converters for receiving respective phases of the AC signal. The output of each converter is fed to a respective transformer. The transformers are effectively connected in series. For each transformer, a pair of diodes are provided to produce a desired output polarity. A purely analogue arrangement is employed to control the converters. Specifically, the input to each converter is stepped down, regulated, and optionally processed by a phase-locked loop to ensure that the result is a sine-wave. The resultant outputs (one per converter) are each multiplied by an error signal, and then input to a respective modulator, which produces a respective pulsed output signal having a proportion of output pulses proportional to its input. The pulsed output signals are used to control respective converters. The error signal is derived from the difference between a reference signal and an output of the series sum of the three converters. This system suffers from a number of disadvantages. Since the control is on the basis of a feedback loop from the output of the sum of the converters, the system will be subject to significant ripple. Also, there is no natural way of limiting the voltage which is applied to the transformers at the outputs of the converters, so that unless the transformers are specified to be tolerant to a wide range of conditionsxe2x80x94which increases their size and costxe2x80x94the proposed system will be unsuitable for high power applications (e.g. delivering an output of several kilowatts). Furthermore, if one of the phases is subject to noise, this effect will be transmitted to the other phases, since the error signal is shared.
U.S. Pat. No. 4,680,689, to Payne et al., proposes a system in which each converter receives a pair of input phases, and after regulation applies it to a respective inductor 24, connected to the center tap of the primary of a respective transformer. A pulse modulator is provided to switch connections to the transformer, to control current flow to be substantially proportional to the instantaneous value of DC input. The outputs of the three transformers are added in parallel. Among the disadvantages of this system are thus that three inductors are required. Also, the system has no mechanism for compensating if the input voltages depart from sinusoids.
The present invention provides a power factor correction module, and more particularly provides a power supply suitable for use with discharge lamps, which do not require a constant voltage as they initially need to heat up, and instead draw a voltage which typically varies between 40 and 2000V.
According to the invention, a power factor corrector module for connection to a three phase input supply comprises three single phase circuits for connection between a respective pair of phases, each circuit comprising a converter. The outputs of the three single phase circuits are connected in series. The module further includes a control operable to vary the duty cycle of the converters substantially in synchronism with the respective phase of the supply. More particularly, the control varies the duty cycle of each of the converters using a respective reference signal synchronized with the input voltage to the corresponding converter, and inversely proportionally to a measured instantaneous input voltage to the corresponding converter.
Preferably, each reference signal is sinusoidal and the control varies the duty cycle of each of the converters proportionally to the square of the respective sinusoidal reference signal. However, other waveforms can be used if the waveform of the desired input current is not sinusoidal.
Provision of a separate circuit per phase of a three phase supply enables the harmonic current requirements noted above to be met. Each circuit draws a sinusoidal current from the supply, substantially in phase with the supply voltage. This circuit design is relatively rugged, and cost effective to manufacture. If the three phase supply has a neutral, this can be used, but it is not essential.
Since the control is on the basis of instantaneous input voltage, control in the present invention is feed-forward rather than feedback, which significantly reduces the risk of ripple in the output.
Furthermore, the converters do not need to measure and store the peak value of the mains. The duty cycle of the converters may be limited to a maximum value to ensure that the operating requirements of the transformer are not exceeded.
In a preferred embodiment, the control utilizes the zero crossing points of the supply voltage as a reference for the generation of the reference signals.
Various converter configurations have been considered for use in implementing the module. Having regard to factors such as simplicity, reliability, device stress, the implementation of the control, the required output voltage variation, parallel/series connection of modules and EMC requirements, a half bridge forward converter topology is preferred. This configuration has a lower device count than a full bridge topology and produces better utilization of the switching devices of the converter. However, other configurations may be used to suit particular circumstances.
Preferably, the duty cycle of each of the three converters can by modified using a single external control signal, so that the sum of the output of the converters is modified proportionally.
Each single phase circuit may include a transformer, and a bridge rectifier to rectify the output of the transformer. The rectifier minimizes copper losses in the transformer. Preferably, a diode is connected across the rectifier output. Combination of a diode in this way with a bridge rectifier ensures that the secondary winding of the transformer does not carry load current during the off time of the converter. This configuration also overcomes the problem of transformer core saturation due to secondary volt time imbalance, as would be the case for full wave rectification. Furthermore, the output diode improves the circuit efficiency during off times.
As noted above, the outputs of the single phase circuits are connected in series. In a preferred arrangement, the module includes a single output filter connected to the combined output of the three single phase circuits. This has the advantage that the output filter sees only a small change in duty cycle due to the summation of the three switching waveforms.
The present invention also provides a power supply for connection to a three phase input supply, the power supply comprising one or more modules of the form described above. Such a modular power supply design enables supplies covering a wide power range to be implemented and reduces the number of different design variants required to do so. For example, to drive an ultraviolet arc tube, the total power required may vary from 3 kW to 36 kW depending on the tube length and type. In a power supply comprising a plurality of modules, the control for the modules preferably comprises three common control circuits, each controlling the same single phase circuit of each module, thereby minimizing the number of control circuits required. If the power supply is designed for driving a gas discharge lamp, it preferably includes a low frequency inverter connected to the combined output of the one or more modules, such that the inverter generates a low frequency square wave at the power supply output.